Gear ratio Q's

[ParadiseMJ]

So can somebody school me on gear ratios. My XJ has an AW4, Dana 35c open diff with 3.55's and 31" tires (29.5) my MJ has a BA10, Dana 44 LSD with 3.07's and the same BFG A/T's.

 

Besides the BA10 what's the best combo.

 

For highwayand/or towing?

For general driving? For wheeling, crawling?

[Philistine]

So can somebody school me on gear ratios.

 

For highwayand/or towing?

For general driving? For wheeling, crawling?

 

I'd start with the geartable at 4lo.com http://www.4lo.com/calc/geartable.htm

 

All of the applications you listed above have their own ideal setup. You have to decide what you want to emphisize. You can't gear for wheeling/crawling and expect to do well just driving around. Is it going to be your daily driver? You may want to gear for better gas milage. Is it a second vehicle for wheeling on the weekends? You may want to gear for better low end torque.

[Eagle]

So can somebody school me on gear ratios. My XJ has an AW4, Dana 35c open diff with 3.55's and 31" tires (29.5) my MJ has a BA10, Dana 44 LSD with 3.07's and the same BFG A/T's.

 

Besides the BA10 what's the best combo.

 

For highwayand/or towing?

For general driving? For wheeling, crawling?

31" tires through 3.73 gears results in exactly the same speed-per-RPM as stock tires with 3.55 gears. IMHO that's a good combination for street driving. For street and light towing, either that combination or 4.10 gears. For rock crawling, 4.10 gears is better and 4.56 or 4.88 preferred.

[ParadiseMJ]

So where does that leave me with 3.07's and 31's behind a BA10?

[Geonovast]

Friends don't let friends drive on 3.07s.

 

3.07 would be fine with like... 23 in tires.

[ParadiseMJ]

Friends don't let friends drive on 3.07s.

 

3.07 would be fine with like... 23 in tires.

 

Great, thanks for the anecdote. What I'm actually looking for is the information. As in why, how, what. Just a straight answer would do.

[Geonovast]

3.07 is just too high of gear. It keeps the motor spinning too slow, and it's harder for it to turn the wheels... therefore less power, and it hurts mileage. The bigger you make the tires, the harder they are to turn. That's why when you go bigger, you need to gear lower.

 

As for rock crawling, the lower the better.

[LoTGoD]

Crawling example: 42" IROK's and 5.38's. Awesome offroad...highish RPM's on the street.

 

Good mix: My MJ has 33" ProComp Xtreme AT's and 4.56 gears. Decent on and offroad ratio

 

Bad mix: My TJ had 36" swampers and 3.73 gears in an auto. Overdrive was not usable on the road at all. Did kind of ok offroad.

[ParadiseMJ]

Thank you, that makes it all alot clearer.

 

Looks like a big part of the restoration is re-gearing.

[Geonovast]

Re-gearing your current axles can get pricey. Might be worth it on your D44 depending on what you're going to, but it'll probably be cheaper and easier to find another D30 with the gearing you want.

 

Gives you a good chance to get a non-CAD axle as well.

[ParadiseMJ]

I would likely swap in a non vac 30 in front. So far I have a whopping $550 into this Jeep which includes a trip to the boneyard for a header panel, tailgate, bedliner, mirrors and various other engine and vacuum system parts. Since I'm doing all the work (except re-gearing) I'm hoping to do the whole thing for whatever I sell the XJ (with 230K miles on it) for.

 

Only 169K orig. miles on the MJ.

[Jeepman]

Here is probably the best Calculator out there...

 

Just punch in what engine/tranny/T-case and your gear ratio and tire size and it will do all the math for you then in the oposing side you can do your changes to see what ratio and or tire or transmission will suit your needs...

 

The key is to figure out your OE numbers first then you know where to go from there with your mods...

 

http://www.grimmjeeper.com/gears.html

[88mjmanche]

the gears in your axle are the same thing as in a mountion bike the low gears 1,2,3, and for low speed more power the higher gears 4,5,6 are for speed you work real hard in the high gears to get moving where low gears would be better butttt its hard to go fast in low gears you pedal like crazy and go like 2 mph

 

 

see what I'm getting at :dunno:

[Eagle]

So where does that leave me with 3.07's and 31's behind a BA10?

Undergeared.

 

In stock trim (running stock tires), highway cruise in 5th gear is around 1650 RPM at (IIRC) 60 MPH. 75 MPH may be about 2000 RPM (again, IIRC). 31s, in terms of revolutions per mile (which is what really counts when you're arguing gearing) are approximately 10 percent larger than stock tires. That means with 3.07s and 31" tires a true 60 MPH (not what the speedo reads if you haven't corrected for the bigger tires) will have you turning 1485 RPM at 60 MPH and 1800 at 75 MPH. You can do it -- I'm currently doing it with my '88 Cherokee -- but it's not pretty. 5th gear is essentially useless, and running up through the gears is like driving an old, tired 4-cylinder MJ with a couple of engines in the back for ballast.

[ParadiseMJ]

... in low gears you pedal like crazy and go like 2 mph

 

...I now see :bowdown:

[freakjeep93]

pretty much make 3.07s go away et something better

[ParadiseMJ]

So where does that leave me with 3.07's and 31's behind a BA10?

Undergeared.

 

So, if I have a D30 front in 4.11 LSD (yard) available I could have a shop re-gear the D44 to 4.11 (local honest guys, I know) for about $__________ (fill in blank).

 

P.S. I have 3.55's on my XJ and a step up to 4:11 seems cheaper if I nab the D30 4.11 for $100. :dunno:

[Pete M]

the labor alone could be $300+ The only way to know for sure is to shop around in your area. Generally you can get a better deal if you bring the axle to them sans truck.

[87Warrior]

I got the rear gear done for $300 in labor on the d44 in my TJ.

 

Went with 5.13s for 35" tires. This set up is about 12% deeper than stock but good for the added weight on the rig.

 

For a MJ that has a 4.0 31s and will see some hauling or wheeling, 4.10s would be my personal choice. In a Jeep a good rule of thumb is to gear deeper and be done with it.

[88mjmanche]

I got the rear gear done for $300 in labor on the d44 in my TJ.

 

Went with 5.13s for 35" tires. This set up is about 12% deeper than stock but good for the added weight on the rig.

 

For a MJ that has a 4.0 31s and will see some hauling or wheeling, 4.10s would be my personal choice. In a Jeep a good rule of thumb is to gear deeper and be done with it.

 

 

he has a good point

the highest speed in usa is 65 anyways and it gives people more time ti see your kick @$$ jeep if your going slow

[Geonovast]

he has a good point

the highest speed in usa is 65 anyways and it gives people more time ti see your kick @$$ jeep if your going slow

 

Most of my trip between WI and MO is 70 mph and I'm sure there's places that are higher.

[88mjmanche]

ooooooooo i thought it was 65 in usa hmmmm I'm WRONGGGGGGG :wall:

[Pete M]

plenty of states with 70. even here in Michigan. :D Out west I think there's some 75mph areas.

[Eagle]

75 is the top legal limit I've seen anywhere, even in the western states.

 

Let's put this in perspective. I have to periodically remind you youngsters that the engine on which both the 4.0L and the 2.5L were based was introduced by AMC in 1964, when the vast majority of the cars they sold were "three on the tree" manual transmissions and overdrives were a VERY rare and expensive option. Using both my 1966 Rambler American and my brother's 1970 Gremlin as examples, the stock drive train resulted in a speed-to-RPM ratio of 24 MPH per 1,000 RPM. Translation: 2500 RPM was exactly 60 MPH, 3,000 RPM was exactly 72 MPH.

 

That's simply what was. Even with wimpy valve springs and hydraulic lifters the engine redlined at 5,000 RPM, so cruising all day at 3,000 RPM wasn't hurting the engines at all. Today, with 5-speeds (and 6-speeds) with tall gears AND overdrive top gears, you guys see factory cruising speeds of under 2,000 RPM at 70 MPH and you think anything over 2,000 RPM cruise is "burning up the engine." Not so.

 

Now, to put all this in perspective for a modified MJ -- when I did my tire & gear spreadsheet 33s were pretty much the biggest tire most people were looking at, and I stopped at 4.88 gears for the simple reason that's the deepest ratio you can get for a Dana 30 front axle. So ==> 33" tires with 4.88 gears, in 5th gear (or overdrive for an automatic) will run 2620 RPM at 70 MPH. Heck, that's right on the torque peak. That's where you WANT to run the engine for maximum efficiency.

 

Don't worry about going "too far" with gearing. Most likely, you can't. Even 30" tires with 4.88 gears will cruise at 2890 RPM at 70 MPH. That's just about equal to what the AMC factory was putting in as stock gearing for decades.

[carnuck]

On my '87 MJ (stock 4.0L BA 10/5, 3.07 gears) I went to 33" tires. 5th gear became useful ONLY on flat highways! I went to 3.73 gears with the 33" tires and it matched the original tire to 3.07 (speedo read perfect)

I drove it that way for several years till I tried 4.10 gears (WAAAAY too low! Lost all top speed and power!) If mine had been geared for 3.55 gears, it would've been a different story. The fuel curve was just right for the 3.73/33" setup (I averaged almost 20 mpg consistently)